The present invention relates to a voltage booster circuit, a power supply circuit, and a liquid crystal driver.
A reduction of power consumption has been increasingly demanded for a portable electronic instrument. A liquid crystal device is generally used as a display device provided in such an electronic instrument, for example.
A high voltage is necessary for driving the liquid crystal device. Therefore, it is preferable from the viewpoint of cost that a liquid crystal driver which drives the liquid crystal device include a power supply circuit which generates a high voltage. In this case, the power supply circuit includes a voltage booster circuit. Power consumption can be reduced by using a charge-pump circuit which generates a voltage boosted by a charge-pump operation as the voltage booster circuit.
The charge-pump circuit (voltage booster circuit in a broad sense) connects one end of a capacitor which stores an electric charge with various voltages using a switch element (metal oxide semiconductor (MOS) transistor, for example), thereby boosting the voltage corresponding to the electric charge stored in the capacitor. Therefore, the electric charge stored in the capacitor during the operation is maintained even if the operation of the charge-pump circuit is terminated.
A liquid crystal which makes up a pixel of the liquid crystal device deteriorates when a DC component voltage is applied to the liquid crystal. Therefore, when terminating the operation of the charge-pump circuit which generates the voltage for the liquid crystal device, the voltage applied to the liquid crystal must be controlled by performing a discharge operation according to a predetermined sequence.
However, when the operation of the charge-pump circuit which generates the voltage for the liquid crystal device is terminated, voltage is applied to the liquid crystal due to the electric charge stored in the capacitor. In a simple matrix liquid crystal device (passive matrix type liquid crystal device) in particular, voltage between a COM electrode and an SEG electrode is directly applied to the liquid crystal. Therefore, the electric charge stored in the capacitor must be discharged when terminating the operation of the charge-pump circuit. Moreover, if the electric charge stored in the capacitor cannot be discharged at high speed, a period of time required until the completion of the sequence is increased. This poses inconvenience to the user who repeatedly performs power-on and power-off operations.